Process and apparatus for measuring the warp tension in looms and the like

ABSTRACT

To measure the tension in a set of threads (10) or a warp (11) in a loom or the like, a vibrating device (20) which is rotatable about an axis (23) is moved into the vicinity of the web of material so that the material is thereby slightly deflected from its straight line course. The vibrating device (20) is set into oscillating movement by an excitation device (30). The frequency (f o ) at which the vibration of the device (20) becomes established is directly dependent upon the tension P in the group of threads or the fabric, and this tension can therefore be determined from the frequency f o . The vibrating device (20) may also be set into its intrinsic vibration at an intrinsic frequency f o  simply by the threads (10) or fabric (11) gliding over its surface (24), and this frequency can be measured by means of a sensor (34) and transducer (35). A particularly advantageous construction of the vibrating device (20) consists of a plate with a notch (26) lying on a knife edge (27). The intrinsic vibrations of the vibrating device (20) may be eliminated by equipping the vibrating device with a counterweight (25) which shifts the center of gravity of the system to the point of intersection of the lines of tension exerted by the set of threads or fabric.

Measurement of the warp tension in looms and the like is an essentialfactor for keeping the tension constant by means of warp tensionregulators. A constant warp tension is essential for the production of aperfect weave. Processes and apparatus have therefore always been in usefor measuring this warp tension and using the measurements as a basisfor control parameters with which to activate devices for controllingthe warp tension.

The earliest method of measuring the warp tension and converting it intoa measuring signal uses a deflecting device operated by the forceexerted by the whole warp, for example on the back rest. Thedisadvantage of measuring the whole warp tension lies mainly in thelarge masses which have to be moved but only produce a relatively smalldeflection of the spring mounted back rest. Another disadvantage is thatthe force measurement is produced by all the warp threads so thatvariations in the warp tension across the width of the warp are notdetected.

Other processes and apparatus have been proposed in which the whole warpis deflected between two fixed points by means of a loaded roller or thelike and the magnitude of the deflection is taken as a measure of thewarp tension. Although such devices could in principle be used overseparate portions of the warp, they invariably constitute anobstruction, especially for servicing and operating the loom. Moreover,none of the processes mentioned above has solved the problem of the socalled zero point constancy.

Processes and apparatus recently proposed are based on setting the warp,preferably partially, into localized resonance vibrations, either bybringing an auxiliary mass into contact with a portion of the warpthreads and causing this system to vibrate or by setting only the warpthreads into vibration. In either case, the thread tension can bedetermined from the resulting resonance frequency and the known mass ofthe warp threads in accordance with the principle of the vibrating cord.The problem of zero point constancy is solved in this process.

Even these systems, however, are not free from disadvantages, at leastwith regard to the complicated structure and therefore high cost of suchvibrating systems. The process fails when attempts are made to use iffor measurements in the whole fabric since the weft threads are liableto vary in density.

The present invention relates to a process for measuring the warptension either on the warp itself and/or in the fabric in textilemachines and the like, characterised by the features given in claim 1.

The invention also covers an apparatus having the features given inclaim 10 for carrying out the process.

Exemplary embodiments of the invention will now be explained with theaid of the description and drawings, in which

FIG. 1 is a first schematic representation of the principle ofmeasurement,

FIG. 2 shows the arrangment of the vibrating device and the warp,

FIG. 3 is a schematic representation of a vibrating device withassociated drive means,

FIG. 4 shows schematically a vibrating device in relation to parts ofthe loom,

FIG. 5 shows the geometrical relationships between the widths of clothand the vibrating device,

FIG. 6 is another representation of the geometric relationships,

FIG. 7 represents a variation of the vibrating device, FIG. 8 showsschematically a vibrating device with sensor,

FIG. 9 shows a vibrating device with selfexcitation and FIG. 10 shows avibrating device with counter weight.

In the arrangement shown schematically in FIG. 1, the warp 10 whosetension is required to be measured or the woven cloth 11 is normallygripped between conveyor devices such as, for example, rear cylindricalrollers 1, 2 and front cylindrical rollers 3, 4. A vibrating device 20is introduced between these two lines. This device executes a rotationalvibration about its axis. It is capable of rotating about its axis butwhen there is a deflection from the straight line it produces arestoring force which is proportional to the deflection and to thetension.

The vibrating device 20 and the warp or cloth thus combine to form aresonance system operating at a resonance frequency defined by theformula: ##EQU1## where ω=angular frequency of the vibrating deviceP=tension in the warp or fabric,

m_(R) =rotational moment of inertia of the vibrating device

a=distance of left supporting line from the adjacent edge of vibratingdevice

b=distance of right supporting line from the adjacent edge of vibratingdevice.

c, d=distances between rollers or edges of vibrating device and axis 23.

Formula (1) which can be derived mathematically shows that the tensioncan be determined from the resonance frequency.

Now that the basic idea of the invention has been explained, theconstruction shown schematically in FIG. 2 will be described. Avibrating device 20 mounted to be rotatable about a central axis 23 liesin contact with the warp 10 or web 11. The web is slightly deflectedupwardly to ensure that the vibrating device will always remain incontact with it. In other words, the web (warp 10 or fabric 11) is underthe influence of the tension P on the vibrating device 20.

The vibrating device 20 may consist of a rotatably mounted plate, andits surface of contact 24 with the web (10, 11) may advantageously beregarded as a wear resistant surface (FIG. 3). This method is suitable,for example, for weaving and finishing processes.

Instead of using cylindrical rollers, the warp or fabric may besupported by parts of the operating machine, such as the warp beam, backrest or breast beam of the loom or squeezing rollers, deflecting rollersof the sizing machine, etc.

In cases where a>c and b>d, formula (1) may be reduced to: ##EQU2## Thisproperty becomes particularly important when the vibrating device 20 isused in a part of the machine where the distance between the line ofcontact and the vibrating device is variable, e.g. in the case ofdeflecting rollers which are radially spring mounted in the sizingmachine. One special case is the measurement of the tension of thematerial on the loom. The breast beam 12 (FIG. 4) is then an accuratelydefined surface of support. On the other side of the vibrating device,on the other hand, the fell of the cloth 13 forms an apparent point ofsupport when the shed is open. The distance between the vibrating deviceand the selvedge, however, is also defined under these circumstances.When the shed is closed, on the other hand, the point of support extendsinto the harness. When the dimensions of the vibrating device are smallin comparison to the distance of the vibrating device to the selvedge orto the harness, then the influence of this variable distance isnegligible.

In that case, the variable factor a is several times greater than c, andthe quotient (in formula (1)) makes only a negligible contribution tothe sum ##EQU3## The possibility of mounting the vibrating device 20 bymeans of a cutting blade 27 and a notch 26 in the vibrating device isshown in FIG. 4. The web 10, 11 in this case holds the vibrating device20 firmly against the blade 27.

The vibrating device should have a substantially greater mass than thefabric. In that case, any difference in the weight of the fabric due todifferences in the weft density do not interfere with the results.

To measure the tension P of the warp 10 or of the web of fabric 11, thevibrating device 20 is activated at its resonance frequency. Devices foractivating mechanical vibrating structures are known. They generallyconsist of a drive member, a back coupling or feed back element and anamplifier. Thus, for example, an electro-mechanical activating device 30shown in FIG. 3 can deflect the vibrating device 20 about its axis 23 bymeans of an electro-magnet. The feed back device may consist of knowninductively, capacitatively, optically or pneumatically operatingdistance meters with amplifiers connected in series therewith. Thedevice may comprise, for example, a driving coil 31 and a feed back coil32 with amplifier 33. The vibrating device then automatically vibratesat the resonance frequency. The frequency f_(o) at which the vibrationof the device 20 becomes established is directly dependant upon thetension P of the web of fabric lying on the device 20 in accordance withformula (1).

Formula (1) is only applicable, however, when the deflecting angle αabout the oscillating plate is very small and the center of rotation ofthe deflecting device is quite close to the fabric (FIG. 5). In othercases, the vibrating movement is no longer perpendicular to the plane ofthe fabric. If the warp 10 or fabric 11 adheres to the vibrating devicedue to friction then changes in length take place in section a and b andstretching forces are therefore produced in the warp or fabric.Additional forces therefore arise which depend on the magnitude of thedeflection so that formula (1) is no longer valid and the measurement offorce is no longer accurate.

This source of error can be eliminated by placing the center of rotationof the oscillating structure at the point of intersection 14 of theimaginary extensions of the lines of force along which the tension inthe warp 10 or fabric 11 acts (FIG. 6). The vibrations are in that caseexactly perpendicular to the plane of the fabric and the fabricundergoes virtually no change in length if the amplitudes of thevibrations are small.

The aforesaid changes in length may also be eliminated by enabling thecenter of rotation of the vibrating device to move in the direction ofthe fabric 10 or 11 instead of fixing its location. An example of thisarrangement is shown in FIG. 7, in which the supporting blade 27 is aleaf spring 29 so that the center of rotation of the vibrating devicecan be deflected. The apparent center of rotation then again lies at thedesired point of intersection of the forces of tension.

Alernatively, the center of rotation may be deliberately placed outsidethe point of intersection 14, as shown in FIG. 8, so that when the warp10 or fabric 11 moves, vibrations are produced by frictional forces(which are exactly constant). The frequency of these vibrations is closeto the resonance frequency. The vibrating system can therefore be setinto vibration without the aid of an additional energizing system, andthe frequency f_(o) at which these vibrations become established may bedetermined from the frequency of the force P by means of a sensor 34 anda transducer 35.

The vibrating device 20 may have rotatably mounted rollers 21, 22 tokeep the friction between the warp or fabric and the vibrating device 22at a minimum (FIG. 9). This measure is advantageously used in sizing andfinishing plants.

Formula (1) again is only accurate when the center of gravity of theoscillating structure lies at the center of rotation, i.e. in thelongitudinal axis 23 (FIG. 10). The center of gravity of the oscillatingstructure consisting of vibrating device 20 and optionally its rollers21, 22 may be moved into the center of rotation by placing acounterweight 25 on the line of symmetry 28 passing through thevibrating device 20.

If the center of gravity of the oscillating structure does not lie atthe center of rotation then the system may vibrate as a pendulum, e.g.at zero force. The result then obtained, however, is only slightlyfalsified if the resonance frequency of the whole system and thefrequency of oscillation of the empty pendulum lie far apart. Moreover,the frequency deviation is constant and can be calculated.

I claim:
 1. Process for measuring the tension of a web of textilematerial such as a group of threads or a fabric, characterized in that avibrating device (20) is provided in the region of and on one side onlyof the web of textile material whose tension is to be measured and thesaid web of textile material is moved over this vibrating device (20),in that the vibrating device (20) is set into vibrations by the web oftextile material gliding over it, which vibrations are converted intoelectric signals of a frequency (f_(o)) by means of a sensor (34) andtransducer (35), and in that the tension (P) of the web of textilematerial is determined from the resonance frequency (f_(o)) of thevibrating device (20).
 2. Apparatus for measuring the tension of a groupof threads or a fabric, characterized in that a vibrating device (20) isrotatably mounted about a central longitudinal axis (23) formed by anotch (26) and a knife edge (27), that the knife edge (27) is mounted atthe end of a unilaterally fixed spring (29), that the group of threads(10) or the fabric (11) is in contact with and deflected from a straightline by the vibrating device (20) and that the vibrating device (20) canbe set into movements of vibration about the aforesaid longitudinal axis(23).
 3. A method for measuring the tension of a textile sheet such as aset of threads or a fabric in a textile machine, the said textile sheetbeing guided between first and second guiding means, said methodcomprising providing an oscillating device on one side only of saidtextile sheet, bringing the said device in contact with the said oneside of said textile sheet, deflecting said textile sheet from astraight line path, and determining the tension of the textile sheetfrom the resonance frequency of the oscillating device.
 4. A methodaccording to claim 3, wherein said oscillating device (20) is set intooscillation about its longitudinal axis by means of a back coupledelectromechanical activating device (30).
 5. A method according to claim4, wherein said textile sheet passes to and from said oscillating devicealong paths having directions which intersect with one another in thevicinity of said oscillating device and which extend at angles to astraight line between said first and second guiding means, and whereinthe true or apparent center of rotation of the oscillating device (20)lies at least approximately at said point of intersection (14).
 6. Amethod according to claim 3, wherein said textile sheet is moved overrollers (21, 22) which are mounted in the oscillating device (20).
 7. Amethod according to claim 3, wherein said textile sheet is passed over asurface (24) of the oscillating device (20).
 8. A method according toclaim 3, wherein said oscillating device (20) is balanced by acounterweight (25).
 9. A method according to claim 3, wherein the movingmass of said oscillating device (20) is large compared with the mass ofthe textile sheet between said first and second guiding means. 10.Apparatus for carrying out the method according to claim 3,characterized in that said oscillating device (20) is rotatably mountedabout a central longitudinal axis (23), that the textile sheet is incontact with the oscillating device (20) and that the oscillating device(20) can be set into movements of oscillation about the aforesaidlongitudinal axis (23).
 11. Apparatus according to claim 10,characterized in that the textile sheet is deflected from a straightline by the oscillating device (20).
 12. Apparatus according to claim11, characterized in that the oscillating device (20) has rotatablymounted rollers (21, 22).
 13. Apparatus according to claim 11,characterized in that the longitudinal axis (23) of the oscillatingdevice (20) is formed by a notch (26) and knife edge (27).
 14. Apparatusaccording to claim 13, characterized in that the knife edge (27) ismounted at the end of a unilaterally fixed spring (29).
 15. Apparatusaccording to claim 11, characterized in that the oscillating device (20)has a wear resistant surface (24) facing the web of textile material.16. Apparatus according to claim 10, characterized in that theoscillating device (20) has rotatably mounted rollers (21, 22). 17.Apparatus according to claim 10, characterized in that the longitudinalaxis (23) of the oscillating device (20) is formed by a notch (26) and aknife edge (27).
 18. Apparatus according to claim 17, characterized inthat the knife edge (27) is mounted at the end of a unilaterally fixedspring (29).
 19. Apparatus according to claim 10, characterized in thatthe oscillating device (20) is balanced about its longitudinal axis (23)by a counterweight (25).
 20. Apparatus according to claim 10,characterized in that the oscillating device (20) is arranged in theregion of an electro-mechanical activating device (30).
 21. Apparatusaccording to claim 20, characterized in that the electro-mechanicalactivating device (30) has an oscillating coil (31), a back couplingcoil (32) and an oscillator/amplifier (33).
 22. Apparatus according toclaim 10, characterized in that a sensor (34) with transducer (35) isassociated with the oscillating device (20) whereby vibrations of theoscillating device (20), which has been activated by said textile sheetto vibrate in its own mode, are converted into electrical signals. 23.Apparatus for measuring the tension in a set of warp threads in a loomcomprising first and second means spaced apart from one another andsupporting said warp threads under tension in the space between saidfirst means and said second means; means for moving said warp threadslongitudinally through said space; an oscillatable thread contactingdevice mounted for oscillating motion about an axis extendingtransversely with respect to the direction from said first to saidsecond means, said thread contacting device contacting said warp threadsfrom one side only of said warp threads and at locations before andafter said axis in the path of movement of said warp threads to deflectsaid warp threads from a straight path extending directly from saidfirst means to said second means; and means for sensing oscillations ofsaid oscillatable device about said axis and determining the frequencyof vibration of said warp threads to provide a measure of the tension insaid warp threads.